System and method for tracking unique visitors to a website

ABSTRACT

A system and method for analyzing traffic to a website is provided that is based on log files and that uses both server-side and client-side information channeled through one source to create a more complete picture of activity to a website. In one preferred embodiment, a sensor code is embedded in a requested web page, and sends information back to the web server where the website resides. This additional information is logged along with normal requests.

This application claims the benefit from U.S. Provisional PatentApplication No. 60/377,616 filed May 6, 2002 whose contents areincorporated herein for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and method for analyzingtraffic to a website.

2. Background of the Related Art

Programs are available for analyzing traffic to a website. One suchprogram is described in co-pending U.S. patent application Ser. No.09/679,297, filed Oct. 4, 2000, entitled “System and Method forMonitoring and Analyzing Internet Traffic”, which is incorporated hereinby reference for all purposes and is assigned in common with the presentapplication. These systems can be generally classified into twocategories: log-based tools and Internet-based tools, with theaforementioned system being an example of a log-based tool.

Log-based tools for analyzing traffic to a website are generallyoperated by the owner of the website or their hosting provider. Thesource of raw data for log-based tools typically comes from the webservers hosting the website being analyzed. As visitors to the websiterequest web pages, files, and embedded content, the web servers hostingthe website are typically configured to automatically make entries intoone or more log files describing each request. Log-based tools will readthese log files as the source of raw data for the analysis.

Internet-based tools, such as that described in U.S. patent Ser. No.09/326,475, entitled “Internet Website Traffic Flow Analysis”, by C.Glommen and B. Barrelet, are generally operated by the owner of the tooland provided as a service that website owners can subscribe to. Togenerate a source of data for the service, the website owner willtypically copy JavaScript code provided by the service provider into thecontent of the website being analyzed. As visitors to the websiterequest web pages, the embedded JavaScript code collects information andthen calls a second web server operated by the service provider,transmitting the collected information.

Both log-based tools and Internet-based tools have their drawbacks. Oneof the drawbacks of log-based tools is that some of the trafficgenerated by visitors to the website may be intercepted by variouscaching systems—designed to improve Internet performance—before thoserequests get to the web server hosting the website. When this happens,the web server hosting the website never receives the request andtherefore, does not make an entry into the log file leaving the dataincomplete. Internet-based tools, on the other hand, benefit from beingtriggered by the visitor's web browser, so that even if the request ishandled by a caching system, the JavaScript code in the content willstill trigger the transmitting of data to the service provider.

One of the shortcomings of Internet-based tools is their inability torecord and analyze requests for non-JavaScript enabled content such asPDF documents and other downloads. Because these file formats do notinclude any JavaScript capabilities, these requests never trigger thetransmitting of data to the service provider. However, log-based toolswill typically see these requests since they are still handled by theweb server hosting the website. In general, Internet-based tools willonly track content that includes scripting abilities such as HTML,whereas log-based tools can see other content requests as well.

One of the difficulties with traditional log based systems is trackingunique visitors, sessions, and loyalty metrics. Being able to uniquelyidentify a new visitor and a new session can be difficult with theincreasing use of proxy systems that can mask IP addresses. And even ifa visitor and session is uniquely identified, scanning potentially hugevolumes of data for previous sessions can be a barrier to calculatingvisitor loyalty.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

An object of the invention is to solve at least the above problemsand/or disadvantages and to provide at least the advantages describedhereinafter.

In view of the above problems in the art, the present invention providesa system and method for analyzing traffic to a website that is based onlog files, that uses both server-side and client-side informationchanneled through one source to create a more complete picture ofactivity to a website. Client-side only systems, such as Internet-basedtools, will miss content requested by non cookie-enabled scriptablebrowsers and content requested for non-scriptable file formats.Server-side only systems, such as prior log-based tools, will misscontent intercepted by caching systems and valuable client-side data.The current invention is a log-based tool that augments the log filewith additional entries from a client-side sensor, creating a morecomplete picture of total activity on a website. Unlike Internet-basedtools that use code to send information to a second web servercontrolled by the service provider, the sensor code of the presentinvention sends the information back to the web server where the websiteresides. This additional information is logged along with normalrequests.

In one embodiment of the present invention, log files containing normalvisitor requests and the sensor code requests are analyzed to create acomplete picture of visitor traffic space. The system and method of thepresent invention preferably uses multiple cookies in the sensor codewith different expirations in order to determine new sessions. Thesystem and method of the present invention preferably uses multipletimestamps to keep track of unique visitors and the time betweenreturning sessions space. The system and method of the present inventionpreferably uses a two dimensional data table to store the number of daysbetween returning sessions and uses this data table to determine thenumber of unique visitors during any date range.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a schematic diagram of a system for analyzing traffic to awebsite, in accordance with the present invention;

FIG. 2 is a flowchart and schematic diagram illustrating the interactionbetween a visitor and a website using the system of FIG. 1;

FIG. 3 is a flowchart and schematic diagram illustrating a preferredcontrol routine for the sensor shown in FIG. 1;

FIG. 4 is a diagram of a preferred embodiment of cookies that are usedby the control routine of FIG. 3;

FIG. 5 is a flowchart and schematic diagram of a preferred controlroutine for the set/update cookies step of in FIG. 3;

FIG. 6 is a partial block diagram and flowchart of a preferredembodiment of the log engine shown in FIG. 1;

FIG. 7 is a partial block diagram of a preferred embodiment of thedatabase shown in FIG. 1;

FIG. 8 is a schematic diagram of a preferred storage architecture forthe visitor-session-path data component shown in FIG. 7;

FIG. 9 is a schematic illustration of a preferred method of the mergeoverlapping data module shown in FIG. 6;

FIG. 10 is a schematic illustration of a preferred method forcalculating unique visitors using the tables from FIG. 7;

FIG. 11 is a schematic illustration of a preferred method forcalculating unique sessions using the tables from FIG. 7;

FIG. 12 is a schematic illustration of a preferred method forcalculating return frequency using the tables from FIG. 7;

FIG. 13 shows example reports created by a preferred embodiment of thereport engine shown in FIG. 1, for the calculations performed in FIG.10, FIG. 11, and FIG. 12;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a system 100 for analyzing traffic to a website, inaccordance with one preferred embodiment of the present invention. Thesystem 100 comprises a sensor 300 and an engine 101. The engine 101preferably comprises a log engine 600, a database 700, and a reportingengine 1000.

The sensor 300 is installed on one or more web pages 110 which are partof a website 120. The website 120 resides on a web server 130 whichdelivers the web pages 110 as they are requested. The web server 130makes entries into a log file 140 for each request received. The logfile 140 is processed by the log engine 600 and the results are storedin the database 700 which can be delivered as reports by the reportingengine 1000.

FIG. 2 shows a flowchart and schematic diagram of the interactionbetween a visitor 200 and the web server 130 in the system of FIG. 1. Avisitor 200 makes a request 210 for the web page 110 belonging to thewebsite 120 FIG. 1). This request 210 is generally made using a webbrowser. The visitor's request 210 may be served by the web server 130,or it may intercepted by a cache/proxy system 220. Caching and proxyingare common technologies used by browsers and Internet service providersto provide increased performance and resource utilization. If therequest 210 is handled by the cache/proxy system 220, the web page 110may be delivered directly from the cache/proxy system 220. Thus, therequest 210 may never be received by the web server 130. Since the webserver 130 makes entries into the log file 140 for each receivedrequest, the request 210 may not be entered into the log file 140 if itis handled by the cache/proxy system 220.

However, the web page 110 that is delivered by either the web server 130or the cache/proxy system 220 contains the sensor 300. When the web page110 is received by the visitor 200, the sensor 300 will make anadditional request 230, which is adapted to go directly to the webserver 130. The additional request 230 will contain information aboutthe original request 210. The additional request 230 is handled by theweb server 130, and a corresponding entry is made in the log file 140.One feature of the present invention is the sending of the additionalrequest 230 back to the original web server 130 where the website 120resides. The request is not sent to a second web server operated by aservice provider. Thus, the log file 140 will contain entries caused byboth original requests 210 and additional requests 230.

FIG. 3 is a flowchart and schematic diagram of a preferred controlroutine for the sensor 300. The control routine is preferably written inJavaScript, which is widely understood by most web browsers. When theweb page 110 is loaded into a visitor's web browser, the control routineof the sensor 300 is executed. The control routine begins by readingexisting cookies in step 310. The contents of the cookies provideinformation on the state of the current visitor, includingidentification and session information. Based on the existinginformation, the cookies are set or updated in step 320. This process isdescribed in more detail below. Next, at step 330, data is collectedfrom the visitor's browser. This data may include client-side browserparameters such as screen resolution.

In the final step 340, the control routine constructs a unique querywhich is sent back to the original web server 130 as the additionalrequest 230. The construction of the unique query preferably includes arandom or unique number and data collected from the browser and cookiesfrom the previous steps. In particular, the unique query preferablyincludes a reference to the original web page 110 that triggered theexecution of the control routine of the sensor 300. In this manner, theadditional request 230 contains information about the original request210 so that the system 100 can determine what the original request 210was even though the original request 210 may not be in the log file 140as discussed previously. The query is preferably sufficiently uniquesuch that it will not be intercepted by the cache/proxy system 220.

FIG. 4 shows a block diagram of one preferred embodiment of the coolies400 that are used by the control routine. The embodiment of FIG. 4 showsthree cookies 400. The first, Cookie-A 401, is a persistent cookie thatis set to never expire or to at least have a very long expiration time.This cookie 401 contains certain variables 410 that provide for uniqueidentification, session determination, and loyalty calculations.Cookie-B 402 and Cookie-C 403 use relative expirations of 30 minutes andzero to allow the detection of a new session to occur. If the visitor200 closes their browser, Cookie-C will expire. If the visitor 200 isinactive for 30 minutes, Cookie-B will expire. It should be appreciatedthat other combinations of cookies and expiration times can be used,while still falling within the scope of the present invention, as longas multiple cookies with varying expirations are used to assist in thedetermination of a new visitor session. The steps within the controlroutine of FIG. 3 will set and update this state information, asdescribed further below.

The preferred variables 410 in the cookies 400 will now be described.The “hash” variable is used to select the appropriate cookies in thecase where multiple sets of cookies 400 appear. Multiple sets can appearif multiple websites have overlapping domains. The “hash” variableprovides an identifier to select the correct set of cookies 400 for thewebsite 120 of interest.

Another purpose of the variables 410 is to provide a means for uniquelyidentifying visitors. This is preferably accomplished by using acombination of the “unique” variable and the “f-time” variable. When avisitor 200 visits the website 120 for the first time, the cookies areinitialized, as will be described in more detail below. During thisinitialization, the “unique” variable is set to a random number and the“f-time” variable to the current time. A combination of these twovariables is used as a key for uniquely identifying visitors. While avariety of variables could be used to build a unique key, one benefit ofthe sensor 300 is that the key is carried by the visitor 200 in thecookies 400 and is not affected by proxy servers which mask IPaddresses.

Another purpose of the variables 410 is to provide a means fordetermining certain loyalty metrics including whether or not the visitoris new or returning and the amount of time that has elapsed since theirlast session if any. To accomplish this, three timestamp variables arepreferably used: “f-time”, “l-time”, and “s-time” that record thetimestamp of the first session, last session, and current session,respectively. The difference between the “l-time” and the “s-time”variables allows the system 100 to determine the return frequency of thevisitor 200. This is described in more detail below, with reference toFIG. 8.

FIG. 5 is a flowchart and schematic diagram of a preferred controlroutine for the set/update cookies step 320 of the control routine ofFIG. 3. The control routine begins in the upper left of the schematicand ends in the lower tight. In the first step 510, it is determined ifall three cookies 400 exist. If they do exist, then only the expirationof Cookie-B is updated in step 520 to thirty minutes in the future.Otherwise, the routine continues to check for the existence of Cookie-Ain step 530. If Cookie-A exists then a series of updates occur at step540: the “l-time” variable of Cookie-A is set to the value of “s-time”,and “s-time” is set to the current clock time of the browser; Cookie-Bis initialized with the appropriate expiration and bash value; andCookie-C is initialized with the appropriate expiration and hash value.If Cookie-A does not exist in step 530, control moves to the series ofupdates at step 550, where all three cookies are initialized. InCookie-A, a unique number is selected for this visitor, and all threetimestamps are set to the current time. The “hash” variable is set to anidentifier of the website domain, and expiration is set to the maximumallowable. Cookie-B and Cookie-C are initialized with appropriateexpirations and hash values. The use of the three variables “f-time”,“l-time”, and “s-time” in calculations is described further below. Anaspect of the current invention is that the cookies 400 are set andupdated within the sensor 300, which generally runs within the browserof the visitor 200. Thus, the cookies 400 are set on the client-side ofthe system 100 instead of the server-side.

Although not shown, the control routine of FIG. 5 could optionally lookat the request itself to see if cookie data has been passed from anotherwebsite. This technique would allow two websites which are linkedtogether to share cookie information.

FIG. 6 is a partial block diagram and flowchart of one preferredembodiment of the log engine 600. In general, the log engine 600 isresponsible for processing the log file 140, which contains informationfrom the web server 130 in response to the requests 210 and additionalrequests 230. In general, as the log engine 600 processes the log file140, it will update the database 700. However, the log engine 600 maycreate reports or summaries directly, while still falling within thescope of the present invention.

The log engine 600 preferably includes three modules: a Unique VisitorIdentification module 610, a Visitor Loyalty Calculation module 620, anda Merge Overlapping Data module 630. These modules will be described inmore detail below, after describing the data structures involved.

FIG. 7 is a partial block diagram of a preferred embodiment of thedatabase 700. The database 700 preferably includes storage for: TotalNew Visitors by Day 710, Total New Sessions by Day 720, a ReturningSessions Grid 730, and Visitor-Session-Path 740. Total New Visitors byDay 710 stores the total number of new visitors that were identified bythe log engine 600 such that, when a new visitor is identified, thestorage element 750 for the day it occurred on is incremented by one.Likewise, the Total New Sessions by Day 720 stores the total number ofnew sessions that were identified by the log engine 600, such that whena new session is identified, the storage element 750 for the day itoccurred on is incremented by one. The Returning Session Grid 730 is atwo-dimensional array that contains elements 750 of storage for eachday. For each day, there are elements of storage for the number “daysago” since the last session. This is used in the loyalty calculationsdescribed further below. The visitor-session-path data 740 keeps recordsfor each unique visitor that link to or include records for eachsession, and the path that was taken through the website 110 during thesession.

FIG. 8 is a schematic diagram of a preferred storage architecture forthe visitor-session-path data 740 component of the database 700. Thisstorage contains unique visitor records 810. Each unique visitor record810 contains or links to a chain of sessions 820 that comprise thevisitor's history, in that the visitor 200 may have one or more sessions820 with the website 120. Each session 820 contains or links toinformation regarding the path 830 that was taken through the website120 by the visitor 200. The path 830 identifies each page 840 or filethat was seen or downloaded by the visitor.

Referring back to FIG. 6, the Unique Visitor Identification module 610uses the information provided by the sensor 300 that was sent in theadditional request 230, to uniquely identify each visitor represented inthe log file 140. The module 610 preferably uses the “unique” and“f-time” variables to create a unique identifier for each visitor.Unique visitor records 810 are stored in the database 700, as shown inFIG. 8. In addition, the module 610 determines when a new visitor hasoccurred and when a new session has occurred. This is accomplished bycomparing the “f-time”, “l-time”, and “s-time” variables, with thecurrent visitor records 810 and subsequent session records 820, if any.

The “f-time”, “l-time”, and “s-time” variables hold the timestamps ofthe visitor's first session, previous session, and current session,respectively. The session records 820 also hold timestamps. Thetimestamps are compared to see if a new session has occurred that is notyet in the database, and possibly a new visitor record altogether.Alternately, the sensor 300 could flag the occurrence of a new sessionwhen certain cookies 400 are expired and reset (FIG. 5). Regardless ofthe technique, if a new session has been identified, the module 610 willincrement the element 750 in the Total New Sessions by Day table 720(FIG. 7), for the day that the new session began. If a new visitor hasbeen identified, the module 610 will increment the element 750 in theTotal New Visitors by Day table 710 for the day that the new sessionbegan.

A preferred implementation of the Visitor Loyalty Calculation module 620(FIG. 6) is triggered when a new session is identified in the previousmodule. Upon detecting a new session, this module uses the “l-time” and“s-time” variables to determine if there was a previous session and howlong ago it was. If the “l-time” and “s-time” variables are the same,then there is no previous session. But, if the “l-time” variable isbefore the “s-time” variable, then the number of days between the twodates is used to increment the Returning Session Grid 730 (FIG. 7). The“day” is calculated from the “s-time” of the new session. The “days ago”is calculated from the difference between the day of the “l-time” andthe day of the “s-time”. The appropriate element 750 in thetwo-dimensional Returning Session Grid 730 is incremented. As will beshown in more detail below, this two-dimensional grid 730 is importantfor creating unique visitor reports.

A preferred implementation of the Merge Overlapping Data module 630(FIG. 6) will properly populate the records of the visitor-session-pathdata 710 shown in FIG. 8. As indicated above, the sensor 300 causesadditional requests 230 to be logged in the same log file 140 that thefirst requests 210 were recorded. This may cause a duplication ofcertain information. The module 630 preferably identifies the entries inthe log file 140 as either coming from the original requests 210 or theadditional requests 230. This can be accomplished by looking at theconstruction of the entry. The sensor-initiated additional requests 230will have a unique identifier.

The Merge Overlapping Data module 630 merges the two types of entriesinto a complete path 830. As shown in the example in FIG. 9, the pathassociated with the original requests 910 for a particular session andthe path associated with the sensor requests 920 are merged to form acomplete path 830. This complete path 830 is the path 830 used for thesession 820 shown in FIG. 8. In the example of FIG. 9, the MergeOverlapping Data module 630 determines that “Page 1” 840 is duplicatedin both the original requests 910 and the sensor requests 920. Themodule 630 only makes one entry in the complete path 830 for these twoentries. Similarly, the module 630 determines that “Page 2” 840 is onlyindicated in the sensor requests, and “Page 3” 840 is only indicated inthe original requests 910. Using the timestamp and possibly the referralinformation of the entries, the module 630 creates a complete path 830that reflects the actual path taken by the visitor.

An aspect of the present invention is the use of both server-side andclient-side information channeled through one source to create a morecomplete picture of a visitor's activity on the website 120. Client-sideonly systems, such as Internet-based tools, will miss content requestedby non cookie-enabled scriptable browsers and content requested fornon-scriptable file formats. Server-side only systems, such as priorlog-based tools, will miss content intercepted by caching systems andvaluable client-side data. The present invention is a log-based toolthat augments the log file 140 with additional entries from aclient-side sensor 300, creating a more complete picture of totalactivity on a website.

FIGS. 10-13 demonstrate some preferred features of the reporting engine1000. The report engine 1000 is preferably adapted to calculate uniquevisitors for any arbitrary date range, and break this calculation intofirst-time visitors and prior visitors. This is not a straightforwardcalculation due to the fact that a visitor may have more than onesession during the selected date range. The Returning Session Grid 730of the database 700 is used in making these calculations. As shown inthe schematic diagram of FIG. 10, first time (1020) unique visitors iscalculated by adding all of the shaded elements 1015 in the Total NewVisitors by Day table 710. The shaded elements 1015 are selected iftheir day falls within the selected date range 1010 for the calculation.Prior (1030) unique visitors is calculated by adding all of the shadedelements 1016 in the Returning Session Grid 730. The shaded elements1016 are selected if their day falls within the selected date range 1010for the calculation and the “days ago” value for the element 1016 isgreater than the distance between the element 1016 and the beginning ofthe date range 1010. This calculation eliminates repeat sessions duringthe date range and only counts each visitor once. Total unique visitorsis simply the addition of first time 1020 unique visitors and prior 1030unique visitors.

FIG. 11 is a schematic diagram of a preferred method for calculatingtotal sessions 1110 and repeat sessions 1120 for any arbitrary daterange 1010. Total sessions 1110 is calculated by totaling all of theshaded elements 1115 in the Total New Sessions by Day table 720. Theshaded elements 1115 are selected if their day falls within the selecteddate range 1010 for the calculation. Repeat sessions (1120) iscalculated by adding all of the shaded elements 1116 in the ReturningSession Grid 730. The shaded elements 1116 are selected if their dayfalls within the selected date range 1010 for the calculation and the“days ago” value for the element 1116 is less than or equal to thedistance between the element 1116 and the beginning of the date range1010.

FIG. 12 is a schematic diagram of a preferred method for calculating areturn frequency histogram 1210. This calculation comes directly fromthe Returning Session Grid 730. Each element in the return frequencyhistogram 1210 is calculated by adding all of the shaded elements 1216in the same row. The shaded elements 1216 are selected if their dayfalls within the selected date range 1010 for the calculation.

By analyzing the ratio between first time 1020 and prior 1030 visitors(FIG. 10), the number of repeat sessions 1120 (FIG. 11), and thereturning frequency 1210 FIG. 12), website owners can begin tounderstand the loyalty metrics surrounding their visitor traffic. FIG.13 shows some example reports for these three calculations. The uniquevisitors report 1310 is calculated for an arbitrary date range 1305. Thereport 1310 displays the total unique visitors 1311 and the portion thatare first time visitors 1312 and the portion that are prior visitors1313. The sessions report 1320 is calculated for an arbitrary date range1305. The report 1320 displays the total sessions 1321 and the portionthat are unique 1322 and the portion that are repeat sessions 1323. Thesession frequency report 1330 is calculated for an arbitrary date range1305. The report 1330 displays the number of sessions 1331 versus thenumber of days ago for the previous session.

While the foregoing description includes many details and specificities,it is to be understood that these have been included for purposes ofexplanation only, and are not to be interpreted as limitations of thepresent invention. Many modifications to the embodiments described abovecan be made without departing from the spirit and scope of theinvention, as is intended to be encompassed by the following claims andtheir legal equivalents.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art. In theclaims, means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

1. A method of analyzing traffic to a website hosted by a server,comprising: sending a web page with an embedded sensor in response to afirst request from a client; and sending, using the embedded sensor, anadditional request, comprising information about the first request, tothe server when the web page is received by the client.
 2. The method ofclaim 1, wherein the additional request is adapted to not be interceptedby a cache/proxy system.
 3. A method of obtaining information about avisitor to a website hosted by a server, comprising: delivering data tothe visitor in response to a first request; and sending informationabout the first request to the server after the data is delivered to thevisitor, wherein the information about the first request is sufficientlyunique so as to not be intercepted by a cache/proxy system.
 4. Themethod of claim 2, wherein the additional request comprises a uniquequery.
 5. The method of claim 4, wherein the unique query comprises: aunique number; data collected from a client browser; and data collectedfrom at least one cookie.
 6. The method of claim 5, wherein the at leastone cookie comprise: a first cookie having a first expiration time; asecond cookie having a second expiration time; and a third cookie havinga third expiration time.
 7. The method of claim 6, wherein the firstcookie has an expiration time that is longer than expiration times ofthe second and third cookies, and the second cookie has an expirationtime that is longer than the expiration time of the third cookie.
 8. Themethod of claim 7, wherein the expiration time of the third cookie iszero.
 9. The method of claim 3, wherein the additional request comprisesa unique query.
 10. The method of claim 3, wherein the unique querycomprises: a unique number; data collected from a client browser; anddata collected from at least one cookie.
 11. The method of claim 10,wherein the at least one cookie comprise: a first cookie having a firstexpiration time; a second cookie having a second expiration time; and athird cookie having a third expiration time.
 12. The method of claim 11,wherein the first cookie has an expiration time that is longer thanexpiration times of the second and third cookies, and the second cookiehas an expiration time that is longer than the expiration time of thethird cookie.
 13. The method of claim 12, wherein the expiration time ofthe third cookie is zero.